1. Field of the Invention
The present invention relates to semiconductors devices and, more particularly, to semiconductor heterostructure barrier (HBV) diodes.
2. Description of Related Art
Semiconductor devices known as varactor diodes are characterized as having a capacitance that varies as a variable bias voltage is applied to a depletion region of the diode. That is, as a reverse voltage is applied to a PN junction of the varactor diode, electrons in the n-region are attracted to the cathode while holes in the p-region are attracted to the anode. The result is a depletion region between the anode and cathode that behaves as the dielectric of the device. As the applied reverse voltage increases, the depletion region (i.e., dielectric) widens, while the capacitance across the PN junction decreases since capacitance is inversely proportional to dielectric thickness. Therefore, by varying the reverse voltage across a PN junction of a varactor diode, the junction capacitance can be varied.
Due to their variable capacitances, varactor diodes are used in a number of different semiconductor applications including, for example, frequency multipliers, harmonic generators, oscillators and phase shifters, etc. Conventional varactor diodes include both P-N junction diodes and Schottky diodes. Schottky diodes include layers that are doped with impurities in order to achieve a desired variable capacitance, and are often used as frequency multipliers for converting a low base-band signal to higher output frequencies. However, Schottky diodes unfortunately create harmonics at both even and odd frequencies, thereby reducing the overall efficiency of this type of multiplier circuit.
Conventional P-N junction diodes and Schottky diodes are also plagued by the presence of leakage current, which lowers the quality of the diode. Another type of variable-capacitance devices is barrier varactor diodes, which minimize this leakage current.
A barrier varactor includes the careful choice of materials and thicknesses to optimize the effectiveness of the barrier therein. For instance, known barrier varactors include heterostructure varactors formed on GaAs substrates using a variety of semiconductor materials, typically resulting in AlGaAs stacks that create a barrier to current flow and act as voltage dependent capacitors. An advantage of heterostructure varactors is that they can be created with symmetric CV characteristics and very low current flow. As such, the use of heterostructure barrier varactors, instead of Schottky diodes, in millimeter wave circuits can eliminate the even harmonics, as well as reduce the need for DC bias and additional idler current sources, thereby enabling a higher efficiency converter. However, the use of heterostructure varactors is limited in its chemistry, and has yet to be easily and efficiently extended into silicon technology.
Accordingly, a need exists in the art for improved methods and heterostructure barrier varactors suitable for use in silicon technology.